Artificial kidney systems are used to treat a patient's blood so as to remove waste products therefrom. One type of dialyzer used in such systems is commonly referred to as a coil dialyzer. It includes a flattened, tubularly-shaped, semipermeable membrane of a material such as cellophane or a regenerated cellulose derivative know as Cuprophan. A predetermined length of membrane is wound along with an appropriate support member about a center core which is then enclosed within a cylindrical housing. Blood from a patient enters the dialyzer through an inlet in the core, flows through the dialyzer inside the membrane and exits from the dialyzer through an outlet. Dialysis solution flows in a crosswise direction through the housing and between the wound support and membrane.
Dialysis solution contacts the membrane and due to the difference in waste product concentration between the blood and the dialysis solution, the waste products, such as urea and creatinine, diffuse from the blood, through the membrane and into the dialysis solution. Also, during treatment of the blood in the dialyzer, water is removed from the blood by virtue of a process known as ultrafiltration. The amount of water which is removed is related to the difference in blood pressure and dialysis solution pressure on opposite sides of the membrane.
After assembly, but prior to sale, the dialyzer is sterilized by flushing steam and a cleansing gas, such as ethylene tri-oxide (ETO), through the dialyzer at temperatures in excess of 120.degree. F. The steam activates any bacteria in the dialyzer so that the bacteria will be destroyed when it takes on or reacts with the ETO. The steam provides a low humidity environment of less than 50% relative humidity.
It has been found that such sterilization causes the cellulose membrane to shrink. Due to the fact that the membrane and the support member may be wound snugly together about a dialyzer central core, shrinkage of the membrane may result in relative movement between the membrane and support member. This relative movement may result in rupture of the membrane and consequently cause the membrane to leak, thereby permitting undesirable blood leakage.
Further, the method by which the membrane is manufactured introduces stresses in the molecular structure of the cellulose material. Shrinkage due to the sterilization and those stresses may weaken the membrane and undesirably impair the consistency of its physiological properties.
In endeavoring to improve the characteristics of cellulose membranes for dialysis, researchers have attempted various types of water treatments. One such treatment involving the process of "water annealing" the membrane for the purpose of increasing hydraulic permeability was discussed in "Proceedings, Seventh Contractors' Conference of the Artificial Kidney Program", 1974, pps. 70, 73. Another water-related treatment suggested successively dampening and drying the cellulose membrane and noting the resultant dimensional changes to determine corresponding density changes (see "Transactions, American Society of Artificial Internal Organs,", Vol. XVI, 1970, pps. 115-120). However, no prior art exists which recognized the problem of cellulose membrane leakage occasioned by sterilization during manufacture.
It is accordingly the principal object of the present invention to provide a method of treating a semi-permeable membrane so as to minimize shrinkage during sterilization so as to, in turn, minimize subsequent damage to the membrane.
This and other objects and advantages of the invention will become clear from the following description of a preferred embodiment of the invention and appended claims.